1. Field of the Invention
This invention relates to the recovery of tall oil from the alkaline pulping of wood from coniferous trees, and in particular, it relates to a method for improving the recovery of tall oil in the step of the process in which tall-oil soap is acidulated to obtain tall oil.
2. Description of the Prior Art
It is known that in the kraft or sulfate pulping process, tall oil can be recovered as a major by-product. Particularly in the pulping of pine or Douglas fir, tall oil, in the form of its sodium salt, is present in the relief liquors which are drawn from the digester. From these relief liquors, also called black liquor, some water is evaporated, and then the liquor is transferred to a skimming tank. The tall-oil soap tends to rise to the surface, and it can readily be skimmed off. A certain amount of tall oil soap can be recovered in that way without further effort. Additional tall-oil soap remains in the black liquor, and the invention of copending application Ser. No. 759,070, filed Jan. 13, 1977 and now U.S. Pat. No. 4,085,000, is concerned with how the recovery of the remaining tall-oil soap can be achieved. In accordance with that invention, an improvement in the recovery of the tall-oil soap is achieved by adding a small but effective amount of a terpolymer having a composition as defined in U.S. Pat. No. 3,873,417.
The usual next step in the recovery of tall oil is the acidulation step, a step in which the recovered tall-oil soap is treated with sulfuric acid to convert the soap to the free-acid form. As was the case with the tall-oil soap, a certain amount of tall oil can be recovered without taking any additional steps; this invention is concerned with how the recovery of additional tall oil from the acidulation mixture can be achieved.